To examine the correlates of heavy Internet use and determine the associations of heavy Internet use with various health risk behaviors and health-promoting behaviors among Chinese adolescents, an anonymous, self-administered health behavior questionnaire was completed by 2427 matriculants into a Hong Kong university (mean age = 18.9 y) and returned at compulsory health examination. Of students, 14.8% reported heavy Internet use (greater than 4 h/day) and such use was associated with lower likelihood of engaging in health-promoting activities such as exercising and seeking medical care. At the same time, heavy Internet use was correlated with multiple risk behaviors such as skipping meals and sleeping late as well as poorer health outcomes such as higher likelihood of being overweight or having hypersomnia. Given the double burden of poorer health outcomes and fewer health-promoting behaviors, heavy Internet users represent a particularly challenging group for adolescent health promotion. (Contains 2 tables.)